The invention relates to an oscillator including a resonator which is coupled to an amplifier stage comprising an amplifier transistor and a load transistor, a load signal path from a collector of the amplifier transistor to an emitter of the load transistor, to which load signal path the resonator is coupled, and a bootstrap signal transfer from the load signal path to the base of the load transistor, which base is coupled to a reference voltage conductor via a bias resistor.
An oscillator of this type is known per se from Japanese Patent publication Kokai 2-21708. FIG. 1 shows the known oscillator. An amplifier stage of the type described in the opening paragraph comprises Q2 as an amplifier transistor and Q5 as a load transistor. The base of the load transistor Q5 is coupled to the positive power supply voltage via bias resistor R7. The load signal path from the collector of Q2 to the emitter of Q5 comprises a load resistor R2. Resonator 15 is coupled to the load signal path. The bootstrap signal transfer from the load signal path to the base of Q5 (point 17) is effected from the collector of Q2 (point 11) and proceeds via transistor Q4, emitter resistors R6 and R5 and transistor Q3 which constitute a differential stage. Together with transistor Q5 and load resistor R2, this differential stage constitutes a local positive feedback loop. Resonator 15, coupling capacitors C2 and C3, driver transistor Q1 and the amplifier stage form part of an oscillation loop.
The known oscillator can start oscillating at a frequency determined by the resonator. To this end, the amplifier stage is to compensate for the signal losses which occur in the resonator 15 and the coupling capacitors C2 and C3 so that the loop gain in the oscillation loop is larger than one.
A practical resonator generally has different resonance frequencies, for example, because the elements of the resonator comprise unwanted reactances such as a self-inductance caused by the connection terminals in a capacitor, or, for example as a result of inductive and capacitive couplings between two different conductors. An illustrative example is a soldering point on a conductor constituting a capacitance of approximately 0.5 picoFarad to a proximate conductor.
Resonator 15 of the known oscillator may be, for example, an LC circuit provided with varicap diodes, used in a UHF television tuner. FIG. 2a illustrates the attenuation T(R) of the positive feedback network in the known oscillator constituted by such a resonator 15 and coupling capacitors C2 and C3. Resonator 15 has three resonance frequencies (f1, f2 and f3) at which the resonator has a maximum impedance and the attenuation T(R) is consequently minimal. The desired resonance frequency is f2, and f1 and f3 are parasitic resonance frequencies. FIG. 2b shows the gain T(A) of the amplifier stage with driver transistor Q1. FIG. 2c shows the resultant loop gain of the oscillation loop T(L).
Outside frequency f2, the known oscillator may also start oscillating at frequency f1 or f3 because the loop gain of the oscillation loop at these frequencies is larger than once. Moreover, the known oscillator may have an unwanted relaxation oscillation. If resonator 15 is removed from the oscillator shown in FIG. 1, the latter will function as a relaxation oscillator. In this case, the coupling capacitors C2 and C3 are periodically charged and discharged by the amplifier stage which behaves as a current switch. If resonator 15 is incorporated in the oscillator circuit, as is shown in FIG. 1, the relaxation oscillation may still occur. This risk notably exists when the loop gain in the oscillation loop at the relaxation oscillation frequency is approximately equal to or larger than that at the desired resonance frequency.
In the known oscillator, the voltage difference between the collector of amplifier transistor Q2 and the power supply voltage is comparatively large. Comparatively large direct currents flow through the load resistor R2 in the load signal path as well as through the bias resistor R7 which couples the base of transistor Q5 to the power supply voltage. To prevent saturation of transistor Q2, the known oscillator requires a comparatively high power supply voltage.
The difference between available power supply voltage and the minimally required oscillator power supply voltage may be so small that the oscillator power supply voltage cannot be taken from a voltage stabilizer. Generally, this is desired. First, an unwanted detuning of the oscillation frequency due to a change of the available power supply voltage is thereby reduced. Secondly, a voltage stabilizer suppresses the transfer of alternating voltages in the available power supply voltage to the oscillator power supply voltage supplied by the voltage stabilizer. Such alternating voltages may modulate the oscillator in an unwanted manner, so that the output signal will comprise unwanted spectral components.